The present application relates to an active material for a rechargeable battery capable of occluding and releasing lithium ions, a rechargeable battery using this active material, and an electronic apparatus using this rechargeable battery.
In recent years, electronic apparatuses typified by mobile phones, mobile information terminal devices (PDAs), or the like have become widespread, in relation to which there is a strong demand for further miniaturization, weight reduction and increased life spans. Accordingly, as a power source, the development of batteries, in particular, compact and lightweight rechargeable batteries capable of obtaining a high energy density is being pursued. Recently, without being limited to the above-described electronic apparatuses, studies are also being carried out into the application of rechargeable batteries for various purposes represented by battery packs, which are removable power sources, electric vehicles such as electric cars, electricity storage systems such as household electricity servers, and power tools such as electric drills or the like.
As rechargeable batteries, ones using various discharge and charge principles have been widely proposed; however, among these, ones using the releasing and occluding of lithium ions or the like are promising. This is because it is possible to obtain a higher energy density than lead batteries, nickel cadmium batteries, and the like.
A rechargeable battery is provided with a positive electrode and a negative electrode, as well as an electrolytic solution, and the negative electrode includes a negative electrode active material capable of occluding and releasing lithium ions or the like. As the negative electrode active material, carbon material such as graphite is widely used; however, recently, as a result of demand for further improvement in battery capacity, the use of Si is being studied. This is because, since the theoretical capacity of Si (4199 mAh/g) is far greater than the theoretical capacity of graphite (372 mAh/g), it is possible to expect a large improvement in the battery capacity.
However, since Si vigorously expands and contracts during charge and discharge, the negative electrode active material is easily cracked primarily in the vicinity of the surface layer. If the negative electrode active material is cracked, since a highly reactive new surface (active surface) is generated, the surface area (reaction area) of the negative electrode active material is increased. In this manner, since the electrolytic solution for forming the coating film derived from the electrolytic solution is consumed in the new surface along with the generation of the decomposition reaction of the electrolytic solution in the new surface, the battery characteristics such as the cycle characteristics are easily decreased.
Here, in order to improve the battery characteristics such as the cycle characteristics, various studies are being carried out in relation to the configuration of rechargeable batteries. Specifically, in order to improve the cycle characteristics and the initial charging and discharging characteristics, a non-crystalline or low crystalline coating portion (SiOy: 0.5≦y≦1.8) is provided on the surface of a core portion (SiOx: 0≦x≦0.5) (for example, refer to Japanese Unexamined Patent Application Publication No. 2011-233497). In addition, in order to fulfil the same purpose, lithium is doped into a silicon-silicon oxide based composite (for example, refer to Japanese Unexamined Patent Application Publication No. 2009-212074).